The present invention relates to the detection and recognition of absent and/or double sheets in the sheet transport paths of printing machines.
It is known in this art to employ capacitive sensing techniques to detect and distinguish between the presence of a single sheet in the sheet transport path, the absence of a sheet in such path, or the presence or two more improperly overlying or stuck-together sheets. For example, Federal Republic of Germany Patent No. 1,816,862 discloses the use of a sensing capacitor responsive to the dielectric character of the transported sheet material. The capacitance changes which the sensing capacitor undergoes in response to variations in the thickness of the sensed sheet material, i.e., due to the absence of the material or due to the improper presence of a double thickness of material, is evaluated in accordance with a technique which involves detuning or changing the resonance frequency of a resonant circuit of which the sensing capacitor forms a part.
The disadvantage of the various known capacitive measuring techniques conventionally employed in this art is that they do not really exhibit a level of accuracy high enough for reliable detection and discrimination of normal sheets, absent sheets, and double or multiple sheets. Furthermore, such systems, especially those which operate at high frequencies and rely upon high-frequency excitation for their operativeness per se, may often exhibit excessive levels of response and sensitivity to external interference or extraneous factors such as variations in sheet moisture content and so forth.